Product packaging

ABSTRACT

A product such as meat or a ready meal ( 12 ) is packaged in a tray ( 14 ) covered with a cover film ( 34 ), these being bonded together by dielectric heating between opposed electrodes ( 18, 24 ). The dielectric heating is carried out through at least one barrier layer ( 22, 28 ) comprising a foam material of a polymeric material that is not susceptible to RF. The film ( 34 ) may be of laminated form, for example comprising a polymer such as APET laminated to a bonding layer of polyethylene. This welding process avoids heat wastage, it can provide narrower seals, and enables a good seal to be obtained despite contamination of the surface of the tray.

This invention relates to a method for packaging a product in a tray,suitable for packaging food, and to an apparatus for performing thismethod.

The packaging of foodstuffs in an aluminium foil tray is known, and somesuch foil trays are coated with a polymer coating. Such a polymer-coatedfoil tray may be covered and sealed by a sheet of a plastic materialthat can be heat sealed to the rim of the tray. The polymer used to coatthe foil is typically polyethylene. To enable heat sealing, the coversheet would typically be of laminated form, incorporating aheat-sealable layer of for example polyethylene. It may also incorporatelayers of other polymers, such as amorphous polyethylene terephthalate(APET), polyvinylchloride (PVC), polyamide (PA), or polyvinylidenechloride (PVdC). Although heat sealing is widely used, it hasdisadvantages, particularly the problems encountered where there iscontamination on the rim, and the inevitable heat losses from the hotwelding elements.

The packaging of foodstuffs in a plastic tray is also widely known, anda suitable material for such a tray is the polyester, polyethyleneterephthalate (PET), which may be either crystalline (CPET) or amorphous(APET). Other tray materials include polypropylene, or polyvinylchloride (PVC). The selection of material for the tray depends upon thenature of the contents, and so on whether the tray has to be heatresistant. For example polypropylene trays can be used for foodstuffswhich are to be microwaved, whereas for foodstuffs to be heated in aconventional oven CPET trays are preferable. Such a tray may be coveredand sealed by a transparent sheet of a plastic material that can be heatsealed to the rim of the tray. And as with an aluminium tray, to enableheat sealing, the cover sheet would typically incorporate aheat-sealable layer of for example polyethylene, and may alsoincorporate layers of other polymers, such as APET, PVC, PA, or PVdC.Similarly, to enable heat sealing, the tray is also typically oflaminated form, with a heat-sealable layer on its upper surface; such aheat-sealable layer may be referred to as a surface bonding layer. Thecost of the cover sheet for such packages would be less, and recyclingeasier, if the surface bonding layer could be omitted. These benefitswould also arise from using a tray with no surface bonding layer.

According to the present invention there is provided a method forpackaging a product, using a tray and a cover sheet, the methodcomprising bonding the cover sheet to the tray by dielectric weldingbetween opposed electrodes, wherein at least one of the electrodes iscovered by a barrier layer comprising a foam material of a polymericmaterial.

The invention also provides an apparatus for performing such a method.

Preferably this foam layer is self-adhesive, so that it can be fixed tothe electrode. Polymeric materials that are not significantlysusceptible to RF include PTFE, polypropylene and polyethylene, but thepreferred material for the foam is acrylic. The suitability of this foammaterial is contrary to previous expectations that barrier materialsshould be non-porous; experience had been that small air gaps sometimesled to sparks and arcing, with consequential damage to the materialbeing sealed. The foam structure does not appear to give this problem.Preferably the polymeric material of the foam structure has a lowdielectric loss factor (which is the product of the dielectric constantand the loss tangent), so it does not get hot; for example values ofloss factor less than about 0.05 are considered to be low, and suchmaterials are not considered to be weldable, while materials with a lossfactor less than 0.01 would be considered to be inactive to RF.

Preferably the foam layer is itself covered with a a non-porous polymerlayer of a polymer that has a low susceptibility to RF, and a suitablematerial for this purpose is a polyimide tape. This too may beself-adhesive.

This dielectric bonding process provides the major advantage that a goodseal can be formed despite the presence of contamination on thesurfaces. This is not usually possible by heat sealing. For example,contamination by fats, oils (such as tuna fish oil), or aqueoussolutions, do not prevent the formation of a good quality seal. It isnot necessary for there to be any surface bonding layer on a polymerictray, or any adhesive.

When dealing with a foil tray, it will be appreciated that the polymericmaterial on the foil, and/or the polymeric material of (or on thesealing surface of) the cover sheet, must of course have a sufficientlyhigh dielectric loss factor that it is heated by dielectric heating. Theuse of dielectric welding to provide the seal provides the majoradvantage that a good seal can be formed despite the presence ofcontamination on the surfaces.

Although the two items are referred to as a tray and a cover sheet, thecover sheet may comprise a layer of foil as well as a layer of polymericmaterial. However, preferably the cover sheet is much more flexible thanthe foil tray. The cover sheet may also comprise a layer of paper orcard, bonded to a suitable polymeric material.

The foam layer is preferably no more than 2 mm thick, and may be about 1mm thick.

The radio frequency supply may in principle be at a frequency between 1MHz and 200 MHz, usually between 10 MHz and 100 MHz, but stringentlimits are imposed on any emitted radio waves. In practice therefore thechoice of frequency may be more limited. For example the supplyfrequency may be 27.12 MHz, or 40.68 MHz.

Preferably the radio-frequency signal generator is a solid-state device,and the signals are supplied via a matching network. The matchingnetwork preferably is an active matching network, incorporating at leastone variable capacitor controlled by a servo motor; it monitors theradio frequency current and voltage, and automatically adjusts the valueof the or each variable capacitor in accordance with variations in theload.

The invention also provides a plant to seal film to trays by thismethod; this may be achieved by modifying a conventional plant.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawings, inwhich:

FIG. 1 shows a cross-sectional view of a welding apparatus for packaginga food product; and

FIG. 2 shows a plan view of a packaging plant.

Referring to FIG. 1, a welding apparatus 10 is shown, partlydiagrammatically, for packaging a food product 12 (such as a ready meal)in a stiff, generally rectangular tray 14 of aluminium foil with asurface bonding layer of polyethylene, that has rounded corners and aperipheral lip 16. The apparatus 10 includes a lower aluminium die 18which defines a generally rectangular aperture 20 into which the tray 14locates, and the upper surface of the die 18 is coated with a 1 mm thickbarrier layer 22 (described below), so that when the tray 14 is locatedin the aperture 20 its lip 16 is supported by the upper surface of thelayer 22 on the die 18. An upper aluminium die 24 has a 5 mm deep recess25 of the same shape as the aperture 20, surrounded by a ridge 26 with aflat lower surface which is also coated with a barrier layer 28.

The barrier layers 22 and 28 both consist of a high-strength VHB (trademark) double coated acrylic foam tape, of thickness 1 mm, stuck to therespective die 18, 24, and covered by a strip of polyimide film such asKapton (trade mark) of thickness 50 μm (0.002 inches). The acrylicmaterial has a dielectric loss factor of 0.028 (at 1 kHz), which is low,while that for the polyimide is 0.0018 (at 1 kHz), so the polyimide isinactive to RF.

The lower die 18 is earthed, while the upper die 24 is connected via aconductor 36 through an active matching network 30 to a solid-state RFgenerator 32. The matching network 30 incorporates variable capacitorscontrolled by servomotors. This ensures that despite changes in theload, the circuit remains tuned to the operating frequency. The earth towhich the lower die 18 is connected is the earth of the matching network30.

In use of the apparatus 10, a tray 14 containing a food product 12 islocated into the aperture 20. A film 34 of amorphous polyethyleneterephthalate (APET) with a coating of polyethylene on its lower surfaceis placed on top of the tray 14, and the upper die 24 is lowered so thatthe film 34 and the lip 16 are sandwiched between the barrier layers 22and 28 on the dies 18 and 24. The generator 32 is then activated (forexample for 1.5 seconds), such that the polyethylene of the film 34 iswelded to the polyethylene coating the lip 16 of the tray 14. The upperdie 24 is then lifted up, and the sealed tray 14 bonded to the film 34is removed.

This sealing process is surprisingly effective, despite the presence ofthe aluminium foil which is a good heat conductor and so acts as a heatsink. The effect of the radio frequency is to generate heat within thecoatings of polyethylene on the foil tray 14 and on the cover film 34,and so to bring about welding at the interface; unlike heat sealingthere is no requirement for heat to diffuse into the polymer, and indeedthe temperature will be at its highest in the vicinity of the interface.The foam barrier layers 22 and 28 may also become warm from the effectof the RF, which helps to reduce heat loss from the materials that arebeing welded.

It should be appreciated that the sealing film 34 may be a multi-layerlaminate, with APET providing strength, and one or more other polymerssuch as ethylene/vinyl alcohol copolymer (EVOH) laminated to it toprovide or enhance particular properties such as oxygen impermeability.For example the film 34 might comprise an upper layer 15 μm thick ofAPET; a 3 μm thick oxygen barrier layer of EVOH; a lower layer 15 μmthick of APET; and a 50 μm thick layer of polyethylene; these layers maybe bonded together by thin layers of adhesive. The sealing process isremarkably effective, and can provide a good seal despite the presenceof contamination on the lip 16, such as traces of blood, fat or oil fromthe product 12.

Although the foil tray 14 is described as having a surface bonding layerof polyethylene, this bonding layer may comprise different organicmaterials, for example being a mixture of polymers, such as a modifiedester of succinic acid of vegetable origin.

It will be appreciated that a welding apparatus may differ from thedescribed above, for example the upper die 24 may be connected to earth,and the lower die 18 be connected to the RF generator 32 via theconductor 36 and the network 30.

The apparatus 10 of FIG. 1 is shown as having a single aperture 20 sothat a single tray 14 can be sealed in one operation. In a modificationa lower die might provide a plurality of apertures so that a pluralityof trays 14 can be supported, the upper die providing a correspondingplurality of projecting flat ridges so that the lips 16 of the trays 14are sandwiched between the dies. In each case the barrier layers 22 and28 are provided on each die, and remain attached to the die throughout amultiplicity of welding operations; they can be removed and replacedwhen they become worn or damaged.

For example, referring now to FIG. 2, there is shown a plan view of aconventional foil tray sealing plant modified so that the sealing can beperformed in accordance with the present invention. The plant 50comprises a continuous belt made up of rectangular aluminium plates 52each defining four apertures 54 to locate trays 14 (as shown in FIG. 1),the plates 52 being linked together by chains 56 on each side of thebelt. Trays are placed in each aperture 54, and are filled with thedesired food product; the plates 52 carrying the filled trays move (inthe direction of the arrow A) step-wise, passing under a covering module58 in which the trays are covered by a cover film, and the cover film issealed to the lip of each tray. The covering module 58 seals the coverfilm to the four trays in one plate 52, this sealing operation taking afew seconds, and then the next plate 52 is moved into position under thecovering module 58.

Conventionally such a tray sealing plant would utilise heated plates andhigh pressure to bring about the sealing. In contrast the presentinvention uses dielectric heating to form the seal. Each plate 52 issupported by an insulating frame 60 of acetal (e.g. Delrin™) strips,those at the end being 25 mm thick, so it is electrically isolated. Eachplate 52 is machined so as to define a 7 mm wide flat-topped rim 62around each aperture 54 which projects 3 mm above the remainder of theplate 54, and this rim 62 is covered with a 2 mm thick acrylic foamlayer covered with polyimide film (as described in relation to FIG. 1).The covering module 58 is also electrically isolated from the remainderof the plant 50 by insulating acetal blocks. The covering module 58 isconnected to the earth of the matching network 30. As in the apparatusof FIG. 1, the covering module 58 includes an upper plate which definesridges that correspond to the rims 62, and these ridges are also coveredwith an insulating barrier layer of 2 mm thick acrylic foam covered withpolyimide film. A copper spring strip (not shown) contacts the undersideof the plate 52 carrying the trays undergoing sealing, the copper springstrip being connected via a conductor 36 and a matching network 30 to asignal generator 32 (as described in relation to FIG. 1).

Thus as each plate 52 moves into position under the covering module 58,the covering module 58 presses the cover film on to each tray, and thelips of the trays and the cover film are sandwiched and compressedbetween the rims 62 and the corresponding ridges in the upper platewhich forms part of the covering module 58. The signal generator 32 isactivated for 2 seconds so that an RF signal is applied between theearthed upper plate forming part of the covering module 58, and theplate 52 which is live. This seals the film to the lip of each tray. Thetotal dwell time may be 5 seconds, so that the welded seal is heldcompressed for another 3 seconds after application of the RF energy. Thepressure is then released and the next plate 52 in the belt moved intoposition.

When packaging food products this plant 50 avoids the inevitable energywastage associated with the use of conventional heated elements. Inaddition it enables a good seal to be formed despite the presence ofwater, fat or other contaminants on the lip of the tray. It does notrequire high pressures compressing the film onto the lip, and forexample a pneumatic air supply at 80 psi (530 kPa) is ample compared toalmost twice that air pressure as used conventionally; this may supplyair to a 6 inch bore cylinder, providing a total load of about 1000 kg.The total sealing area for each plate 52 may for example be about 100cm². It has been found to provide better results in burst tests thanconventional heat sealing, and the proportion of sealed trays wherethere is a leak is significantly less than with conventional heatsealing: the proportion of leakers can be less than 1%, whereas withconventional heat sealing it may be as many as 5%, and as much as 20%where the food product includes a sauce (because traces of sauce on therim prevent heat sealing). Because the heat is generated within thepolymeric material, the trays and their contents are not significantlyheated, so there is a much shorter curing time (as the welded materialcools down), and so the welding process is markedly quicker than withheat sealing. Furthermore the quality of the seal is not significantlyaffected even if the trays are subjected to a rapid freezing procedureimmediately following the welding step. If the strength of the seal isto be adjusted, for example to enable the film to be peeled offsubsequently, this may be achieved by changing the power supplied by thesignal generator 32.

Because of the improved seal produced by the present invention, thewidth of the seal can be reduced from the conventional value of about 6mm down to 3 mm; indeed the width could be further reduced to less than1 mm, for example 0.5 mm or less. This enables smaller rims to be used,saving material, transport costs and shelf space. Indeed the toolingprofile (of the opposed electrodes) may be modified so as to create aseal that is designed to fail at a particular location around the rim,for example to avoid the need to puncture the film before heating thecontents in a microwave oven.

Although the invention has been described above in relation to sealing afilm to an aluminium foil tray, it will be understood that it is equallyapplicable when sealing a film to a polymeric tray, for example ofpolyethylene terephthalate.

1. A method for packaging a product, using a tray and a cover sheet, themethod comprising bonding the cover sheet to the tray by dielectricwelding between opposed electrodes, wherein at least one of theelectrodes is covered by a barrier layer comprising a foam material of apolymeric material.
 2. A method as claimed in claim 1 wherein the foamlayer is self-adhesive.
 3. A method as claimed in claim 1 wherein thefoam layer is of acrylic foam.
 4. A method as claimed in any claim 1wherein the foam layer is itself covered with a non-porous polymerlayer.
 5. A method as claimed in claim 1 wherein the tray comprisesaluminium foil.
 6. An apparatus for packaging a product, using a traywith a rim, and a cover sheet, the apparatus comprising a die defining arecess to locate the tray, the die acting as an electrode, and anopposed electrode whereby the rim and the cover sheet can be locatedbetween the die and the opposed electrode to be subjected to dielectricwelding, the apparatus also comprising means to supply RF signalsbetween the die and the opposed electrode, wherein at least one of theelectrodes is covered by a barrier layer comprising a foam material of apolymeric material.
 7. An apparatus as claimed in claim 6 wherein thefoam material is an acrylic foam.
 8. An apparatus as claimed in claim 6wherein the foam layer is itself covered with a non-porous polymerlayer.
 9. A method of dielectric welding using opposed electrodes,wherein at least one of the electrodes is covered by a barrier layercomprising a polymeric foam material.
 10. A plant for packaging foodproducts in trays, wherein each tray is supported in an aperture in aplate, wherein each plate is electrically insulated from the remainderof the plant, the plant also incorporating a radio-frequency signalgenerator, means to connect the plant to earth and means to connect theplate to the signal generator, so the trays are welded to a cover filmby dielectric welding, wherein the dielectric welding is carried outthrough a barrier layer comprising a foam material of a polymericmaterial.